To meet the large demand for cooked ham it has become common to use other large muscle parts of the pig in ham manufacture, for instance, de-limbed shoulder and neck. These pieces of meat are packed into an elastic net prior to cooking. During cooking, the former pieces of meat turn into a compact netted ham with an appealing surface structure due to the netting imprint. In order not to damage the ham surface upon removal of the elastic net it has become common to introduce an edible collagen film between the net and the pieces of meat. In the course of the cooking process the collagen film becomes an integral component of the ham and the elastic net can easily be removed from the finished product without doing harm to the meat surface.
The preparation of edible collagen films has been described, for example, in DE 642 922 or DE 1997 0403. Their application as edible wrappings for food has been addressed, for instance, in DE 19 45 527 and their use in wrapping meat products is known from U.S. Pat. No. 3,014,024.
U.S. Pat. No. 5,885,634 teaches that the extensibility and the tear strength (“resistance”) of a collagen film are key factors with respect to the machinability of collagen films. It also teaches how to improve their extensibility without compromising their resistance.
Collagen films with functional ingredients incorporated into the film are known from DE-PS 970 263 (film containing soluble colorants and/or soluble aroma components) and WO 95/17100 (collagen film with finely ground spices embedded as integral ingredients).
Also, an edible collagen film suitable to simplify the manufacturing process for “Black Forest Ham” or other foodstuffs with a black outer coating has been described (U.S. Pat. No. 6,224,919).
Thus, edible collagen films have found wide application in meat and poultry industry permitting more effective manufacturing processes and providing tastier and more appealing products.
Nowadays, collagen films are provided either on “rolls” (continuous film reeled on a cylindrical core; typical film lengths: 50 m or 100 m; typical film widths between 380 mm and 620 mm) or in the form of “sheets” cut to the dimensions required by the customer. A typical wall thickness of the films available ranges between 15 μm and 30 μm (when the film has a moisture content of 10%-15%). Preferably, the thickness of these films is not expressed in terms of “μm” but rather in terms of basis weight (g/m2). For the currently available collagen films the basis weights range between 22 g/m2 and 40 g/m2 (at an absolute humidity of 10%-15%).
The most typical industrial application of collagen films is the manufacture of netted cooked hams addressed above. In general, rolls of edible collagen film are used on applicators similar to the ones addressed in U.S. Pat. No. 4,910,034. On such devices the film is pulled from the roll, guided over a forming shoulder and formed into a tubular film by loosely overlapping the edges of the flat film. The tubing thus obtained is guided through the annular channel between two concentric tubes at the end of which it meets a tubular elastic netting supplied from the outer tubing. In general, the sandwich-structured tubing formed by the elastic netting (outside) and the collagen film (inside) gets closed by means of a clip to cushion the pieces of meat which are delivered through the inner of the concentric tubes which serves as the stuffing tube. When the meat forced through the stuffing tube is cushioned at the end of the latter, it exerts a thrust on both the collagen film and the overlying elastic netting which, thus, are automatically enveloping the meat portion. After closing the sandwich-structured wrapping at the other end by means of a second clip, an ovoid netted product is obtained. This simplified description of the stuffing process is elucidated in more precision in U.S. Pat. No. 4,910,034.
However, despite of the advantages related with the use of edible collagen films exhaustively described, for example, in the patents cited before, one problem has always remained unsolved: air may become trapped between the collagen film and the meat in the course of the wrapping procedure and/or steam generated during subsequent cooking may do the same. As a consequence, air pockets may form, leaving unappetising “craters” on the surface of the finished product. Such products must then be downgraded, resulting in substantial loss of profit.
With the introduction of specific stuffing equipment (e.g. Handtmann-Piereder PX-94 NC) which permits higher productivity and more precise portioning than equipment used in the art before, the problem of air inclusion was recently observed to become even more pronounced and a solution to the problem had to be found urgently.
Various approaches have been made in the past to overcome this air pocket issue. One strategy still applied in today's practice is to manually puncture the netted product subsequent to the stuffing process. In a slightly different design of the same strategy, the stuffed product is either rolled over some kind of spike board or, in a more “semi-automatic” way, the filled netted product rolls down an inclined plain which is spiked. The punctures introduced into the collagen membrane allow the escape of trapped air or steam during the ensuing cooking process. However, in the course of puncturing, the delicate collagen film is observed to tend to split and frequently air pockets remain non-punctured.
From sausage casing industry it has been known for many years that using pre-punctured casings represents a more reliable solution than posterior puncture after stuffing (G. Effenberger, Wursthullen-Kunstdarm, Herstellung-Eigenschaften-Anwendung, Hans Holzmann Verlag, Bad Worishofen, 1991). Effenberger's publication referred to robust, non-edible tubular sausage casings which are known to be significantly more resistant than the fragile collagen flat films are. For that reason, puncturing of those casings did not require sophisticated perforation technologies like, for instance, laser or corona treatment.
The strategy of using a perforated product to solve the problem of air pocket formation in ham manufacture was addressed some years ago in EP 0 711 321 which suggests the use of a perforated collagen film. The patent teaches the larger the perforations are, the more they tend to reduce the strength of the collagen film to an unacceptable level. To avoid reduction of mechanical strength beyond an acceptable level, perforation should be arranged in a pattern which maximises the distance between adjacent perforations. Therefore, the perforated collagen film disclosed in EP 0 711 321 is perforated such that “each perforation is spaced 20 to 100 mm from its closest neighbour”. In a preferred embodiment of that patent “the holes are spaced 30 to 90 mm apart in the longitudinal direction and 16 to 60 mm apart in the transverse direction”. However, it was now observed that such a product would not fulfil industrial requirements, since in practice such distance between neighbouring perforations is not close enough to allow the trapped air to escape efficiently. This may be the reason why a product according to EP 0 711 321 has never solved the problem.
The idea of using a perforated product in ham manufacture was recently revived by promoting a non-edible flat film based on cellulose fibers, bearing an easy-peel coating on one side (product name SUN F, marketed by Unipac Packaging Products Ltd.). The most important feature of the film was its perforation and its machinability. Under the microscope the perforations looked irregularly shaped, which points to a mechanical perforation technology. The holes were spaced by about 10 mm from their nearest neighbors. The problem of air inclusion between the meat and the film was solved by this product, because the initially trapped air was able to escape through the perforation holes. However, due to its paper-like nature the film showed some other features in practice which made it fail. As the film was not edible, it had to be removed from the ham along with the netting. When the easy-peel-coating was not perfect, the film stuck to the meat and the surface of the product was disrupted. Even if the film performed perfectly in terms of avoiding air pockets and being reasonably peelable, the surface of the ham would still look little appealing for lack of surface gloss and due to the very weak netting imprint. This unsatisfactory netting imprint was a consequence of the limited film extensibility (“non-elasticity”) which did not allow the film to smoothly fit the dome shaped surface protuberances generated by the tight-fitting net. On top of this, the surface of the meat was interspersed with macro and micro wrinkles caused by the stiffness of the paper-like product. This made the resulting hams look unattractive. Finally, the color of the surface of smoked hams did not match the traditional color of hams prepared in collagen films which the final customers are used to.